EZ-Set pools seem nice, with the air doughnut... 12'x36" for $98...
We might put a 12'x3' pool inside a 16'x16'x8'tall greenhouse
with 5 $7 doubled 1x3 bows on 4' centers...

And cover the pool surface with a 9'x2" foamboard disk supporting a $60
300'x1" flat pipe spiral, with a 12'x12'x2" layer of foamboard over that,
and mulch around the pool up to the foamboard, with whitewash over (how
much of?) the north wall of the greenhouse. The 2x2'(12'+16') = 112 ft^2
of mulch surronding the horizontal collector would act as a parasitic air
heater, warming the air around the collector and reducing its loss.

Nick